1. Field of the Invention
This invention relates to metallic coatings for internal surfaces and, more particularly, to a method and powder mixture for applying a metallic coating to the internal surface of an article from the coating powder mixture which generates a coating vapor.
2. Description of the Prior Art
During high temperature operation, turbomachinery blading members such as turbine blades, vanes, nozzles, etc., which are fluid cooled such as through use of air in internal fluid-cooling passages, have experienced oxidation and sulfidation reactions on the internal surfaces of such cooling passages. Therefore, it has been recognized that there is a need to apply an internal coating to such components which generally are made of superalloys based on the elements Ni or Co or both.
Coating of such internal surfaces through the use of a fluid which contacts the inner surface while the fluid is passed through an article to be coated has been described for example in connection with such patents as U.S. Pat. No. 4,031,274-Bessen issued June 21, 1977. In addition, the coating of such internal surfaces through the use of a slurry or a powder mixture held within the internal portion of an article and in contact with the surface to be coated have been described in such U.S. Pat. Nos. as 3,900,613-Galmiche et al, issued Aug. 19, 1975 and 4,208,453-Baldi issued June 17, 1980.
Although such known methods can be used to apply various coatings to the internal surfaces of certain articles, it is difficult to control coating deposition when moving fluid through the complex air-cooling passages of a turbomachinery blade member, as well as to provide adequate thickness or to obtain the type of coating desired for a high temperature application. In the case of wet and dry pack coating materials required to be applied against the surface to be coated, known materials are difficult to remove because they have a tendency to agglomerate or sinter together. Accordingly, the removal of such material from within a complex array of passages within an air-cooled turbomachinery blading member is very difficult. Retention of such pack material within the member after coating can inhibit or block cooling fluid flow through the member.